[JimP]

Interesting thread.

There are several things at work on the original poster's photos.
Although the pictures are 5 megabites (which had to be software upscaling), you find quite a bit of edge enhancement which shows up on hard edges. Also consider that to display it on these web pages, it had to be downscaled to fit. If you're usually taking pictures for the web, it makes for a cleaner picture to shoot it closer to the resolution you intend to post unless you can shoot in camera raw and have access to software to resize it to the needed size.